This invention relates to core drilling apparatus that has a corebreaker mounted in a core lifter case for limited movement relative thereto for breaking a core and retaining the core in an inner tube as the inner tube is retracted to the drilling surface.
In U.S. Pat. No. 1,720,700 to Stone there is disclosed one embodiment of a non-axial slit core lifter ring that has an exterior surface that in addition to being circular relative to its central axis is rounded in an axial direction (rounded in a direction perpendicular to its circular curvature about its central axis). The lower shoulder of the recess in which the core lifter is mounted has a high point to cause tilting of the core lifter when the inner tube assembly is retracted. Due to the axial rounding of the ring surface the core lifter can prematurely rotate about horizontally extending axes prior to starting the core taking operation and also increases production costs. Such premature rotation, at times may create a condition blocking the desired entrance of core into the core lifter and/or may have excess rotation during withdrawal and allow slippage of the core in the lifter with possible loss of core. It appears the upper and lower annular edges of the ring are parallel relative to one another. U.S. Pat. No. 790,331 to Terry discloses one embodiment of a cockable core lifter having an exterior surface similar to that of Stone and a second embodiment that has a frustoconical exterior surface.
U.S. Pat. No. 3,540,537 to Brown discloses a core lifter case that has a gradually tapered axial inner surface and a resilient split steel ring having a smooth cylindrical, transverse inner surface that extends substantially the axial length of the core lifter. The central axis of the core lifter, the inner tube and the core lifter case are equally radially spaced from the lifter radial inner surface. Further the core lifter has a smooth, transverse outer axial conical surface of a taper to form a mating fit with the above mentioned surface of the case and a smooth cylindrical, transverse inner surface; and axially opposite axially outwardly and inwardly opening slots that open to the respective terminal edge of the core lifter.
U.S. Pat. No. 2,490,512 to Deely discloses a core lifter case having a frustoconical recess in which a corebreaker is mounted for limited axial movement. The first embodiment of the corebreaker that has a top annular edge that is axially spaced from the lower edge of the inner tube and inclined relative thereto when no core extends into the lifter, is inclined upwardly relative to its lower annular edge, is axially split and is movable while core is being taken to have its upper edge extend horizontally. Thus when the core lifter moves from its datum position to the position its upper edge is horizontal, the lifter moves to have the lifter axis oppositely inclined relative to the central axis of the drill bit from that when the lifter was in its datum position. The exterior and lower part of the interior surface of the core -lifter are of frustoconical shapes and the upper interior surface is cylindrical, the inner surface being toothed. U.S. Pat. No. 1,848,453 to Zublin et al shows additional forms of cockable core lifters in a drill bit.
U.S. Pat. No. 3,428,138 to Capser et al discloses offset core lifter apparatus having a core lifter case with an annular groove that has an inner edge that is inclined relative to the central axis of the core barrel inner tube.